Portable Safes need to be examined once picked up, and after you hit the space key you'll get the chance to attempt unlocking it. Each button on the safe needs to be hit so that the lights above activate in an anti clockwise order. Its hard to explain but really easy once you see what I'm talking about. It is essentially trial and error until you manage to open the safe.
EDIT - It seems there is no consistency for the solutions, so there seems to be no way to look up a solution online, you'll have to figure it out for yourself.